Since polyethylene terephthalate has excellent chemical stability, the use thereof has rapidly increased in the field of life-related materials such as a fiber, film and resin, and foods such as a bottle for drinking water and carbonated beverage. However, the treatment of polyethylene terephthalate having been used and products with disqualified quality that are generated in a large amount along with the increase in the use amount as described above constitutes large social problems.
For the above problem, so-called chemical recycle is effective, in which polyester waste is converted to monomers, which are recovered to be used again as raw materials for polymerization reaction to produce and reuse polyethylene terephthalate. In the method, basically, the reuse loop of the compound with no loss is possible, and reutilization of resources becomes possible. That is, these kinds of collected polyester waste are subjected to depolymerization reaction with ethylene glycol, then to ester exchange with methanol, and the product is further separated and purified to be collected as dimethyl terephthalate and ethylene glycol. Consequently, the effective utilization of resources becomes possible, to result in lowering the cost when taking the step of the reutilization into consideration.
However, when treating polyester waste by the chemical recycle, in the case where polyester waste, in particular beverage bottles are selected as the object to be collected, the removal of foreign matters such as caps and labels is necessary. An example that refers to such a problem and takes measures thereto is shown (see Patent Document 1).
In addition, when fibers and the like from processes other than a polyester production process are selected as the object to be collected, there is such a case where the interfusion of polyester fibers containing dye can not be avoided. Dyes contained in these colored polyester fibers are thermally decomposed in a series of reactions such as depolymerization at high temperatures in the presence of a catalyst, which are then dispersed in components to be collected, to significantly degrade the quality of the component to be collected. An example that refers to such a problem and takes measures thereto is shown in Patent Document 2 below. But, although the inclusion of foreign matters other than polyester in the depolymerization reaction liquid prevents the transfer of the liquid to the subsequent process to cause process troubles, the document does not show any specific example on an effective method for separating and removing these foreign matters.
(Patent Document 1) JP-A-2004-277639
(Patent Document 2) JP-A-2004-217871